In order to solve the increasing environmental and the fuel economy problems caused by a huge number of automobiles, catalyzers for processing the automobile exhaust gas have been extensively installed therein. However, in order to obtain the maximum benefit of a catalyzer, its temperature has to be monitored exactly and constantly by using a temperature sensor having a long-term stability in the high temperature exhaust gas.
Defining the thermal characteristics of temperature sensor more specifically, a temperature sensor having a long-term resistivity change less than .+-.20% has to be employed in this case.
The material of a thermistor to be incorporated in the temperature sensor should be able to withstand the highest temperature of 1000.degree. C. and it should be operable within a temperature range from 300.degree.to 700.degree. C. Therefore, a Mg(Al,Cr).sub.2 O.sub.4 system material has been developed and used. The term "thermistor" is also referred to herein as a "thermistor element".
FIG. 1 shows a cross-section of a conventional temperature sensor showing a thermistor element 1 mounted at its tip. This type of thermistor element 1 can be obtained by inserting platinum pipes or tubes 2a and 2b into the thermistor element 1 and by afterward sintering the thermistor element 1 at a high temperature.
Then, thus obtained thermistor element 1 is sealed in a heat resistant cap 4 constituting a catalyzer temperature sensor shown in FIG. 1. At this time, the platinum tubes 2a and 2b are welded to lead-tubes 3a and 3b in which lead-wires to connect the thermistor element 1 to a proper external device are disposed.
However, in the temperature sensor having the above construction, the metal atoms constituting the heat-resistant cap 4 tend to react with the oxygen atoms occluded in the cap 4. Therefore, in order to compensate the partial pressure of oxygen therein, oxygen has to be supplied from the thermistor element 1. Moreover, the reduction gases such as hydrogen and carbon monoxide gases generated in the cap 4, tend to combine with the oxygen gas in the thermistor element 1, changing the composition of the thermistor element 1.
Thus, as a result of this, the long-term thermistor resistance stability which should be kept within .+-.20% is lost, and the exact determination of the catalyzer temperature becomes impossible.